The present invention relates to wafer cleaning methods, in general, and to a method for cleaning silicon wafers prior to a thermal treatment to minimize the effect upon the minority carrier lifetime.
The preparation of single crystal silicon wafers typically involves the steps of growing a single crystal ingot, slicing the ingot into wafers, lapping, etching and polishing the wafers. Depending upon the required specifications of the electronic device fabricator, the silicon wafers may additionally be subjected to a thermal processing step such as oxygen donor annhilation annealing, thermal processes to control oxygen precipitation, low temperature CVD oxidation, epitaxial deposition, polysilicon deposition and the like.
In such thermal processing steps, the silicon wafers are exposed to a temperature of at least about 300.degree. C. for a duration of at least about one second. Under these conditions, metals such as iron which are present on the surface of the wafer can be driven into the silicon crystal lattice where they can degrade bulk silicon minority carrier lifetime. Ideally, therefore, the silicon wafer surface should be metal-free when it is subjected to a thermal processing step.
In many applications it would additionally be preferred that the silicon wafers subjected to the thermal processing step have a hydrophilic surface. To date, however, this has not been practical. Various limitations associated with the conventional processes used to grow hydrophilic surface layers of silicon oxide have rendered it impractical to grow a hydrophilic silicon oxide layer having a concentration of less than 1.times.10.sup.11 atoms/cm.sup.2 of metals such as iron and chromium. Consequently, wafers are routinely stripped of their oxide layer prior to thermal processing. This approach, however, is not without its disadvantages; silicon wafers having a hydrophobic surface layer can be prone to localized contamination.